Modern automobile engines require high octane gasoline for efficient operation. Previously lead had been added to gasoline to increase the octane number. However, with the removal of lead from the gasoline pool due to environmental concerns, other methods for increasing the octane number are needed. The addition of oxygenates, such as methyl-t-butyl ether (MTBE) and ethanol, may be added to gasoline to increase the octane number. However, MTBE, while generally less toxic than lead, has been linked to ground water contamination. At the same time, some of the high octane components normally present in gasoline, such as benzene, aromatics, and olefins, must also be reduced. Obviously, a process which will increase the octane of gasoline without the addition of toxic or environmentally adverse substances would be desirable.
For a given carbon number of a light naphtha component, the shortest, most branched isomer tends to have the highest octane number. For example, the branched isomers of hexane, monomethylpentane and dimethylbutane, have octane numbers that are significantly higher than that of n-hexane, with dimethybutane having the highest RON. Likewise, the branched isomer of pentane, methylbutane, has a significantly higher RON than n-pentane. By increasing the proportion of these high octane isomers in the gasoline pool satisfactory octane numbers may be achieved for gasoline without additional additives. Adsorbents, such as zeolite 5A, which are selective for the least bulky, lowest RON isomers are known and have been employed in commercial processes following an isomerization operation in order to separate the high RON isomers from the low RON isomers present in light naphtha. In a different approach, U.S. Pat. No. 5,107,052 describes a process for increasing the octane number of gasoline by isomerizing the C5 and C6 paraffins and then selectively adsorbing the dimethylbutane using a molecular sieve selected from the group consisting of AIPO4-5, SAPO-5, SSZ-24, MgAPO-5, and MAPSO-5. In each of these approaches the high RON isomers recovered are added to the gasoline pool to increase the octane number. The low RON isomers which are recovered separately may be recycled to the isomerization operation.
Two methods for calculating octane numbers are currently being used, the Motor-method octane number (MON) determined using ASTM D2700 and the Research-method octane number (RON) determined using ASTM D2699. The two methods both employ the standard Cooperative Fuel Research (CFR) knock-test engine, but the values obtained are not identical. Sometimes the MON and RON are averaged, (MON+RON)/2, to obtain an octane number. Therefore, when referring to an octane number, it is essential to know which method was used to obtain the number. In this disclosure, unless clearly stated otherwise, octane number will refer to the RON.
For the purpose of comparison, the isomers of hexane and pentane have the following RON's:
n-pentane61.7methylbutane92.3n-hexane24.82-methylpentane73.43-methylpentane74.52,2-dimethylbutane91.82,3-dimethylbutane101.0
In this disclosure, the isomers 2-methylpentane and 3-methylpentane will be collectively referred to as monomethylpentane. Likewise the isomers 2,2-dimehtylbutane and 2,3-dimethybutane will be collectively referred to as dimethylbutane. The monomethyl isomer of pentane will be referred to as methylbutane. The isomers of C5 and C6 paraffin are included in the light naphtha fraction of the gasoline pool. One skilled in the art will recognize that some isomers of C7 paraffin may also be present in the light naphtha fraction, however, since heptane and its isomers are generally only present in minor amounts, they will be ignored in the following discussion of the present invention.
Gasoline is generally prepared from a number of blend streams. Gasoline blending streams typically have a normal boiling point within the range of 0 degrees C. (32 degrees F.) and 260 degrees C. (500 degrees F.) as determined by an ASTM D86 distillation. Feeds of this type include light naphthas typically having a boiling range of from about 15 degrees C. to about 70 degrees C. (about 60 degrees F. to about 160 degrees F.); full range naphthas, typically having a boiling range of about C5 to 180 degrees C. (355 degrees F.), heavier naphtha fractions boiling in the range of about 125 degrees C. to 210 degrees C. (260 degrees F. to 412 degrees F.), or heavy gasoline fractions boiling at, or at least within, the range of about 165 degrees C. to 260 degrees C. (330 degrees F. to 500 degrees F.), preferably from about 165 degrees C. to 210 degrees C. (330 degrees F. to 412 degrees F.). In general, a gasoline fuel will distill over the range of from about room temperature to 260 degrees C. (500 degrees F.). The gasoline pool typically includes butanes, light straight run, isomerate, FCC cracked products, hydrocracked naphtha, coker gasoline, alkylate, reformate, added ethers, etc. Of these, gasoline blend stocks from the FCC, the reformer and the alkylation unit account for a major portion of the gasoline pool. FCC gasoline, and if present, coker naphtha and pyrolysis gasoline, generally contribute a substantial portion of the pool sulfur.
Gasoline suitable for use as fuel in an automobile engine should have a RON of at least 80, preferably at least 85, and most preferably 90 or above. High performance engines may require a fuel having a RON of about 100. Most gasoline blending streams will have a RON ranging from about 55 to about 95, with the majority falling between about 80 and 90. Obviously, it is desirable to maximize the amount of methylbutane and dimethylbutane in the gasoline pool in order to increase the overall RON. The present invention is directed to this problem.
ATS zeolites are a molecular sieve having 12-ring pores. In general ATS-type zeolites include silicate-series crystalline microporous materials, such as crystalline alumino-silicates, crystalline metallo-silicates, and crystalline metallo-aluminosilicates having the ATS structure which is exemplified by SSZ-55. SSZ-55 is described and its preparation is taught in U.S. Pat. No. 6,475,463 B1. This reference also teaches its use as an isomerization and hydrocracking catalyst. However, the ability of SSZ-55 to preferentially adsorb methylbutane and dimethylbutane as compared to the other isomers of C5 and C6 paraffins has not been previously described and makes it possible to operate isomerization and hydrocracking operations in a highly efficient mode which was not recognized in the prior art.
As used in this disclosure the words “comprises” or “comprising” are intended as open-ended transitions meaning the inclusion of the named elements, but not necessarily excluding other unnamed elements. The phrases “consists essentially of” or “consisting essentially of” are intended to mean the exclusion of other elements of any essential significance to the composition. The phrases “consisting of” or “consists of” are intended as transitions meaning the exclusion of all but the recited elements with the exception of only minor traces of impurities.